Refactor pagination DB API to return concrete type

 Refactor recent events func to use pagination func

nuke-room-from-db.sh: fix deletion from search table

There is no reason to return a tuple of tokens when the last token is
always the token passed as an argument. Changing it makes it consistent
with other storage APIs

This makes it easier to document what is being returned by the storage
functions and what some functions expect as arguments.

This also removes a cache that is unlikely to ever get hit.

Refactor /context to reuse pagination storage functions

nuke-room-from-db.sh: added postgresql option and help

notifications: Convert next_token to string according to the spec

Fix metrics that have integer value labels

Currently the parameter is serialized as an integer.

Signed-off-by: Konstantinos Sideris <sideris.konstantin@gmail.com>

Make 'unexpected logging context' into warnings

Fix logcontext leaks in rate limiter

Fix 'Unhandled Error' logs with Twisted 18.4

fix http request timeout code

Refactor event storage to prepare for changes in state calculations

Make Client-Server API return 403 for invalid token

Escape label values in prometheus metrics

This gets two arguments, not one.